The present invention relates to an anisotropic conductive film, and more specifically to an anisotropic conductive film comprising a polycarbodiimide copolymer as a material of a film substrate.
In recent years, anisotropic conductive films have been widely used as connector materials to electrically connect electronic components, such as a semiconductor element (IC chip), and circuit boards.
Conventionally-known anisotropic conductive films are formed by dispersing conductive microparticles in a film substrate made of an insulating resin. However, this type of anisotropic conductive film poses structure-related problems, including difficulty in fine pitch connection with the connection target and necessity for a convex (bumpy) terminal of the connection target, e.g., electrode of semiconductor element. Accordingly, WO98/07216 proposes an anisotropic conductive film having a plurality of conductive paths insulated from each other and penetrating a film substrate made of an insulating resin in the thickness direction of the film substrate, as an anisotropic conductive film capable of solving these problems and permitting fine pitch and bumpless structures.
Anisotropic conductive films are normally used for two purposes. One is the use as a so-called mounting connector, wherein an anisotropic conductive film is placed between an electronic component and a circuit board and thermally compression-bonded to the both, thereby to electrically and mechanically connect the electronic component and the circuit board. The other is the use as a so-called testing connector that enables a functional test of an electronic component, wherein an anisotropic conductive film is inserted between the electronic component and a circuit board, and pressure connected to the both to ensure electrical continuity between the electronic component and the circuit board.
An anisotropic conductive film is used as a testing connector to avoid poor yields and increased costs of circuit board, which occur when a functional test of an electronic component is conducted after mounting the electronic component on a circuit board, because a combination of a faulty electronic component and a good circuit board is wasted as being useless.
When an electronic component (semiconductor element or the like) is mounted on a circuit board using an anisotropic conductive film, good electrical continuity between the electronic component and the circuit board cannot be achieved unless the film substrate of the anisotropic conductive film softens or melts sufficiently to firmly adhere to both the electronic component and the circuit board. In recent years, an increasing number of film substrates composed of a polycarbodiimide resin have been used as an anisotropic conductive film for a mounting connector. The reasons for this are low water absorption of polycarbodiimide resin, highly reliable moisture resistance of an anisotropic conductive film employing a polycarbodiimide resin as a film substrate and possible storage thereof at normal temperature. However, the film substrate (polycarbodiimide resin) of said anisotropic conductive film is softened or becomes molten sufficiently only by thermal compression bonding at a temperature of 250xc2x0 C. or higher. On the other hand, it has been found that a base substrate (resin substrate) of a circuit board of a semiconductor device (finished product with a semiconductor element mounted on the circuit board), obtained by thermal compression bonding at such a temperature is deteriorated and discolored, and causes connection failure between said semiconductor device and the mother board.
For performance inspection of electronic components, a functional test (burn-in test) may be conducted at a high temperature of 150xc2x0 C. or higher to assess the function of the electronic components when they generated heat during actual operation. However, almost all the conventional anisotropic conductive films do not have heat resistance to endure high temperatures of 150xc2x0 C. or higher. As a result, the film substrate may soften or melt, partly adhering to a circuit board (testing tool), and prevent continuous testing. In other words, the circuit board may be no longer available for repeated use as a testing tool.
An example of an anisotropic conductive film having superior heat resistance is one having a film substrate formed with silicone rubber. In a test using such an anisotropic conductive film at a temperature of 150xc2x0 C. or higher, however, gaseous silicon may be generated from the film substrate and contaminate the electronic component and/or the circuit board to prevent electrical continuity during the test, or the Si component may adhere to the electronic component (semiconductor element) and/or the circuit board to cause an error.
In view of the above circumstances, it is a first object of the present invention to provide an anisotropic conductive film capable of ensuring good electrical continuity between an electronic component and a circuit board by thermal compression bonding at a low temperature at which the circuit board is not deteriorated. It is a second object of the present invention to provide an anisotropic conductive film wherein, in a functional test of an electronic component at a temperature of 150xc2x0 C. or higher, its film substrate does not adhere to the electronic component or a circuit board, the film ensures electrical continuity between the electronic component and the circuit board with a relatively low pressure (low load), and the film does not generate an out-gas.
With the aim of accomplishing the above objects, the present inventors conducted extensive investigations and found that a polycarbodiimide copolymer, wherein a polyalkylene carbonate unit is introduced to the polycarbodiimide molecule, is capable of softening or melting at relatively low temperatures without affecting the excellently low water absorption of a polycarbodiimide resin, that an anisotropic conductive film comprising a film substrate composed of this copolymer can adhere to both an electronic component and a circuit board at relatively low temperatures by thermal compression, that a thermal setting product of said copolymer possesses heat resistance to temperatures of 150xc2x0 C. or higher and is relatively flexible, and that an anisotropic conductive film comprising a film substrate composed of this thermal setting product is capable of ensuring electrical continuity between the electronic component and the circuit board simply by press adhering the film to the electronic component and the circuit board with a relatively low pressure (low load), thereby enabling testing of the both.
Accordingly, the present invention provides the following:
(1) An anisotropic conductive film comprising a film substrate made of an insulating resin and a plurality of conductive paths insulated from each other and penetrating the film substrate in the direction of thickness, wherein said film substrate is mainly composed of a polycarbodiimide copolymer having a structure represented by the formula (I) below:
R3xe2x80x94NCN"Brketopenst""Parenopenst"R2xe2x80x94NCN"Parenclosest"nR2xe2x80x94A"Parenopenst"R1xe2x80x94Oxe2x80x94COxe2x80x94O"Parenclosest"mR1xe2x80x94Axe2x80x94R2"Brketclosest"x"Parenopenst"NCNxe2x80x94R2"Parenclosest"nNCNxe2x80x94R3xe2x80x83xe2x80x83(I)
wherein m represents an integer of 2-50; n represents an integer of 1-30; x represents an integer of 1-10; A represents a urethane bond; R1 represents an alkylene group; R2 represents an aromatic diisocyanate residue; and R3 represents an aromatic monoisocyanate residue.
(2) The anisotropic conductive film of (1) above, wherein the polycarbodiimide copolymer has a glass transition temperature of 50-150xc2x0 C.
(3) The anisotropic conductive film of (1) above, wherein, in the formula (I), the alkylene group represented by R1 is a hexamethylene group, the aromatic diisocyanate residue represented by R2 is a tolylene diisocyanate residue, and the aromatic monoisocyanate residue represented by R3 is a p-isopropylphenyl isocyanate residue.
(4) The anisotropic conductive film of any of (1) through (3) above, which is used for mounting an electronic component, and which is disposed between an electronic component and a circuit board and bonded to allow adhesion to the both by thermal compression, so that said electronic component and the circuit board are mechanically and electrically connected.
(5) An anisotropic conductive film comprising a film substrate and a plurality of conductive paths insulated from each other and penetrating the film substrate in the direction of thickness, wherein said film substrate is mainly composed of a setting product of a polycarbodiimide copolymer having a structure represented by formula (I) below
R3xe2x80x94NCN"Brketopenst""Parenopenst"R2xe2x80x94NCN"Parenclosest"nR2xe2x80x94A"Parenopenst"R1xe2x80x94Oxe2x80x94COxe2x80x94O"Parenclosest"mR1xe2x80x94Axe2x80x94R2"Brketclosest"x"Parenopenst"NCNxe2x80x94R2"Parenclosest"nNCNxe2x80x94R3xe2x80x83xe2x80x83(I)
wherein m represents an integer of 2-50; n represents an integer of 1-30; x represents an integer of 1-10; A represents a urethane bond; R1 represents an alkylene group; R2 represents an aromatic diisocyanate residue; and R3 represents an aromatic monoisocyanate residue.
(6) The anisotropic conductive film of (5) above, wherein the polycarbodiimide copolymer setting product has a glass transition temperature of 150-250xc2x0 C.
(7) The anisotropic conductive film of (5) above, wherein, in the formula (I), the alkylene group represented by R1 is a hexamethylene group, the aromatic diisocyanate residue represented by R2 is a tolylene diisocyanate residue, and the aromatic monoisocyanate residue represented by R3 is a p-isopropylphenyl isocyanate residue.
(8) The anisotropic conductive film of (5) above, wherein the film as a whole has an elastic modulus of 1 to 1000 MPa at 150xc2x0 C. to 250xc2x0 C.
(9) The anisotropic conductive film of any of (5) through (8) above, which is used for functional testing of an electronic component, and which is inserted between the electronic component and a circuit board and adhered to the both by compression, so that said electronic component and the circuit board are testably conducted with each other.